The exhaust gas from internal combustion engines such as automobile engine contains nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbon (HC) and the like, and these substances are purified by an exhaust gas purifying catalyst and then released into air. As a representative exhaust gas purifying catalyst used therefor, a three-way catalyst where a noble metal such as platinum (Pt), rhodium (Rh) and palladium (Pd) is supported on a porous metal oxide carrier such as γ-alumina is known.
In order to allow for efficient progress of oxidation of CO and HC and reduction of NOx by the activity of the three-way catalyst, the air-fuel ratio in the internal combustion engine must be a theoretical air-fuel ratio (stoichiometric ratio). When the internal combustion engine is operated in the condition of excess air for fuel (lean) or in the condition of excess fuel for air (rich), the three-way catalyst cannot satisfactorily exert its purifying effect.
Therefore, for absorbing the fluctuation of oxygen concentration in the exhaust gas and thereby elevating the exhaust gas purifying capacity of the three-way catalyst, a material having an oxygen storage capacity (OSC) for storing oxygen when the oxygen concentration in the exhaust gas is high, and releasing oxygen when the oxygen concentration in the exhaust gas is low, is used for the exhaust gas purifying catalyst. A representative material having OSC is ceria (CeO2).
It is known that the OSC of ceria is enhanced when ceria is used together with zirconia to form a ceria-zirconia composite oxide (see, Japanese Unexamined Patent Publication No. 8-103650, 8-109020, 8-109021, 2000-319019, and 2001-89143). By this formation of ceria into a composite oxide, the heat resistance of ceria is improved, but sufficiently high heat resistance is not yet obtained.
In Japanese Unexamined Patent Publication No. 10-194742, zirconium hydroxide is used as the zirconia source for improving the heat resistance of the ceria-zirconia composite oxide.
In Japanese Unexamined Patent Publication No. 10-258232, a catalyst for exhaust gas purification having improved sulfur-poisoning resistance and heat resistance is provided. The catalyst comprises a core part made of alumina/titania complex oxide and NOx storage material, and a catalyst-carrying layer thereon made of a metal oxide complex comprising alumina and a catalytic noble metal.
The present invention provides an exhaust gas purifying catalyst having OSC and an improved heat resistance, a metal oxide particle, and a production process of the metal oxide particle.
According to studies in recent years, it has been found that the ceria not only has OSC but also, by virtue of its strong affinity for platinum, prevents particle growth (sintering) of platinum supported on ceria and in turn reduction of the exhaust gas purifying capacity. Therefore, if ceria is used in the form of a ceria-zirconia composite metal oxide as in conventional techniques, the platinum is randomly supported on both ceria and zirconia constituting the composite metal oxide surface, and the ceria cannot fully exert its ability of preventing platinum from sintering.
The present invention provides an exhaust gas purifying catalyst and metal oxide particle, having an effect of preventing platinum from sintering as well as heat resistance and OSC, and a production process of the metal oxide particle.
Incidentally, the above-described problems are partially solved also by the zirconium particle having a ceria surface which is proposed by the present inventors in Japanese Unexamined Patent Publication No. 2004-141833 (published after the priority date of the present application).